An MTA bus driver was busted for assault in Brooklyn this morning after punching a passenger who spit in his face, authorities said.

Robert Volkens, 42, was driving a Q35 bus and got into a fight with a 23-year-old passenger, whose metrocard was not working, when he tried to get on the bus near Kings Plaza around 7:45 a.m., cops said.

The man asked Volkens if he could board the bus without paying but the driver refused to let him on, cops said. After a back and forth another passenger graciously paid the man’s fare, cops said.

The ungrateful passenger wound up spitting in the bus driver’s face twice when he was exiting the bus on Nostrand Avenue near Avenue H, cops said.

Volkens followed him off and punched him, before getting collared for misdemeanor assault, cops said.

“Appropriate action will be taken upon the conclusion of a thorough investigation,” said spokesman Kevin Ortiz.

The victim was treated for minor injuries at Kings County Hospital, police said.